1. Field of the Invention
The present invention relates to a method for preparing an oxide superconducting thin film on a substrate, and more particularly, it relates to a method for preparing a Y-Ba-Cu-O oxide superconducting thin film.
2. Description of the Prior Art
Superconductivity, which is explained as phase transition of electrons, is a phenomenon by which a conductor loses all resistance to electric current under specific conditions and exhibits complete diamagnetism. In a superconductor, electric current of high density permanently flows with absolutely no power loss. For example, losses of about 7%, which are typical in power transmission, could be greatly reduced if superconductive power transmission were employed. Further, the availability of superconductive electromagnets for generating strong magnetic fields will facilitate nuclear fusion reactions, which is said to require electric power in excess of generated energy for development, and in MHD power generation, electric motors, etc. used in the field of power generation, for example.
Further expected is application of superconductors to a power source for a magnetic levitation train, an electromagnetically driven ship or the like, as well as for NMR, pimeson apparatus and high-energy experimental apparatus, etc., and use in the fields of instrumentation and medical care.
In addition to the aforementioned uses in large apparatus, superconductive materials can be adapted to provide various types of smaller superconducting devices. A typical example of such superconducting devices is that employing the Josephson effect, through which a quantum effect is macroscopically developed by applied current when superconductive materials are weakly joined with each other. A tunnel junction type Josephson device, which has a small energy gap between superconductive materials, is expected to be used as a switching element of extremely high speed and low power consumption. Further, application of the Josephson device to a supersensitive sensor for magnetic fields, microwaves, radioactive rays or the like is also expected. Further, as the degree of integration of an electronic circuit is improved, power consumption per unit area approaches the limit of cooling ability. Thus, development of a superconducting device for a very high speed computer is necessary.
In spite of various efforts, superconduction critical temperatures T.sub.c of conventional superconductive materials remained below 23 K, that of Nb.sub.3 Ge, for many years. However, it was discovered at the end of 1986 that sintered bodies of K.sub.2 NiF.sub.4 type oxides such as [La,Ba].sub.2 CuO.sub.4 and [La,Sr].sub.2 CuO.sub.4 superconduct at higher critical temperatures, thereby opening the possibility of non-cryogenic superconductivity. It has been observed that these substances superconduct at a critical temperatures of 30 to 50 K, which are much higher than those of the conventional materials. Some of these ceramic substances superconduct at temperatures exceeding 70 K. However, such superconductive materials, being sintered bodies which may contain unreacted particles, and which tend to be heterogeneous in composition and structure, cannot be directly applied to electronic devices. Such materials must be provided in thin film form with adequately controlled composition and structure for application to electronic devices.
Further, it has been impossible to apply superconducting thin films heretofore developed, which have low critical current density values (J.sub.c) of several hundred A/cm.sup.2, to practical devices. Manufacture of an elongated superconducting member by evaporation of a superconductive material onto a wire or rod, or a tape-like member of metal or the like, is required. Thus, use of an evaporation technique is required for the superconductive material in such manufacturing process.